The invention relates to a device for supporting a rotatable body in a direction parallel to an axis of rotation of the body by means of a fluid bearing comprising two cooperating bearing surfaces which extend transverse to the axis of rotation and which are present on the body and on a support member of the device, respectively, one of the bearing surfaces being pivotable about two mutually perpendicular pivot axes which extend perpendicular to the axis of rotation and which intersect one another in a point of intersection of the axis of rotation and the relevant beating surface.
The invention also relates to a positioning device provided with a device according to the invention.
A device of the kind mentioned in the opening paragraph is known from U.S. Pat. No. 3,841,720. The known device comprises a rotatable shaft which is supported in the axial direction by a spiral groove bearing, the bearing surface of the support member being provided with a pattern of spiralling grooves. The support member is pivotable about the two pivot axes through the use of a support surface in the shape of a sphere segment which is rotatable in a cavity of a frame of the device corresponding to the support surface and whose center coincides with the point of intersection of the axis of rotation and the beating surface of the support member. The use of the support surface in the shape of a sphere segment renders the bearing surface of the support member self-adjusting, i.e. the beating surface of the support member automatically adjusts itself to the bearing surface of the shaft, so that the two bearing surfaces lie in parallel planes during operation.
Owing to manufacturing tolerances, the bearing surface of the shaft often is not exactly perpendicular to the axis of rotation. This deviation from perpendicularity of the bearing surface, which is difficult to avoid, causes a slight, comparatively fast swinging movement of the shaft bearing surface relative to an axis perpendicular to the axis of rotation during the rotation of the shaft. A disadvantage of the known device is that the beating surface of the support member has a difficulty in following the swinging movement of the shaft bearing surface because the self-adjusting capability of the bearing surface of the support member is limited by the friction and the accompanying hysteresis between the support surface in the shape of a sphere segment and the said cavity of the frame. An undesirable vibration of the shaft in axial direction arises from this.